It has been known that cyclohexane is oxidized to yield a mixture of cyclohexanol and cyclohexanone and the mixture is used as a raw material for producing adipic acid (e.g. supervised a translation by Teruaki Mukaiyama, “Industrial organic chemistry-main raw materials and intermediates-”, 1st Edition, TOKYO KAGAKU DOJIN CO., Ltd., December 1978, p. 229-230).
As various oxidation by-products are produced other than desired cyclohexanol and cyclohexanone by oxidizing cyclohexane in a liquid phase, industrially, the reaction is generally carried out so that the conversion rate of cyclohexane may seldom be raised but the selectivity of desired cyclohexanol and cyclohexanone may be raised. Nevertheless, substantial amount of oxidation by-products are produced and therefore, washing the oxidation reaction mixture with water or alkaline water to separate to a mixture containing cyclohexanol, cyclohexanone and cyclohexane and a wastewater containing oxidation by-products is conducted.
Most of oxidation by-products contained in the wastewater discharged by washing are adipic acid and hydroxycaproic acid. Among them, a part or all of adipic acid can be recovered by conducting crystallization treatment of the above-mentioned wastewater, for example, under acidic condition and used efficiently. However, because of poor demand for hydroxycaproic acid as it is, it was usually disposed by incinerating without recovering. As for hydroxycaproic acid, a method for converting hydroxycaproic acid contained in the wastewater to adipic acid by oxidizing the wastewater with oxygen in the presence of platinum group metal catalyst such as palladium is proposed (e.g. JP 2000-103760 A1).